A white LED has been used in an application such as illumination. However, heat generation from an LED package becomes a problem in connection with achieving high power. In the case where an epoxy resin is used for a sealing material, avoidance of yellowing due to the heat generation becomes quite difficult, and therefore a silicone resin has been used for the sealing material of the white LED in place of the epoxy resin. The silicone resin used for the LED is generally classified into two kinds including a phenyl silicone resin and a methyl silicone resin.
The phenyl silicone resin generally used therefor has a high refractive index and a good light-extraction efficiency. Moreover, the resin has a high gas barrier property and also a good adhesion with a package, and therefore has excellent reliability such as moisture-absorption reflow resistance and heat cycle resistance. However, while the resin is superior to the epoxy resin in resistance to thermal yellowing, the resin is far from sufficient in order to respond to achievement of high power of the LED.
The methyl silicone resin is superb in the resistance to thermal yellowing, but has a low refractive index, and therefore has a poor light-extraction efficiency from the LED. Moreover, the methyl silicone resin is mainly composed of dimethyl silicone, and therefore has a low gas barrier property, and also a poor adhesion with the package, and thus has had a problem of being easily peeled during moisture-absorption reflow. In a case where peeling is caused, brightness of light generated from the LED decreases, and therefore such a case is not preferred.
Further, a high power LED appears, and particularly when a package size is small, heat is locally accumulated in a resin portion, and a problem of causing cracks has come up. In a current-carrying test at a high temperature using the high power LED, a temperature in the resin portion is reputedly arrived at a high temperature region of 200° C. or higher, and therefore long-term reliability in the higher temperature region has been required.
In the high-temperature region, in the phenyl silicone resin generally used therefor, brightness degradation by yellowing is significant, and also the cracks are caused by resin deterioration. In the dimethyl silicone resin, although the brightness degradation by yellowing is low, the deterioration of the resin progresses in the high-temperature region, the cracks are caused, and the brightness is degraded, and therefore the resin is inapplicable to an application of the high power LED in several cases.
As described above, characteristics required for an LED sealing material becomes increasingly tough. Therefore, eager wish has been expressed for the sealing material that can respond to the high power of the white LED and achieve both the high refractive index and heat resistance, and further for the thermosetting resin composition having a balance regarding all of the moisture-absorption reflow resistance, the heat cycle resistance and so forth.
A silsesquioxane material having excellent heat resistance and UV resistance attracts attention, and the LED sealing material using such a material has been reported.
Patent literature No. 1 discloses an LED sealing material formed of a thermosetting resin composition by a thermosetting resin having cage silsesquioxane into which an SiH group is introduced, and organopolysiloxane having an alkenyl group.
Patent literature No. 2 discloses a thermosetting resin composition using imperfect cage silsesquioxane as commonly referred to as a double decker. The silsesquioxane is a compound obtained from hydrolytic condensation of phenyltrimethoxysilane, and a position of an Si-Ph group thereof is not random and is subjected to structure control, and therefore the silsesquioxane has excellent heat resistance and light resistance even with the high refractive index.
Patent literature No. 2 discloses a thermosetting resin containing an SiH group and a vinyl group as obtained from a reaction between a compound having a modified SiH group in a base of a silanol group of imperfect cage structure silsesquioxane, and organopolysiloxane having an alkenyl group. Then, a hardened material obtained by hardening the thermosetting resin is described to have the high heat resistance even with the high refractive index, and further a good adhesion with a polyphthalamide resin base material or a silver base material, being an LED package material.